This invention relates to bi-directional communications systems in which a master terminal is linked with a multiplicity of remote terminals over a communication network. More particularly, this invention concerns a remote level adjustment system, for use with a bi-directional, multi-terminal communications system, in which a master terminal sets the output signal level of each of a plurality of remote terminals.
The field of bi-directional, mutli-terminal communications systems includes a system wherein a master terminal initiates and controls communications with a plurality of remote subscriber terminals over a common, shared communications channel. Such systems are designed to have the master terminal supervise and monitor activities which occur at the remote terminal sites, with each remote terminal providing data to the master terminal regarding transactions at its respective site. The data can be provided by a remote terminal in the form of a data word produced in response to a request from the master terminal.
A method for organizing the communications in a bi-directional, multi-terminal communications system comprises the well-known polling technique. According to the technique, the master terminal produces a continually cycling sequence of data words, with each word addressed to an individual remote terminal. Each terminal, upon recognizing the word addressed to it, responds with a reply word. The replies from the remote terminals are detectd by the master terminal in an order corresponding to the polling sequence. The polling technique provides an organized method of high-speed communications which allows the master terminal to continually and rapidly update the status of each remote terminal.
One way of interconnecting the master terminal and the remote terminals is by way of an electrical cable network of the type well known in, for example, the cable TV (CATV) industry. Bi-directional multi-terminal communications systems employing cable television distribution networks are taught in Stewart, et al., U.S. Pat. No. 3,846,703, Oakley, et al., U.S. Pat. No. 3,886,454, and Schrock, et al., U.S. Pat. No. 4,343,042. As is well understood by those skilled in the art, a CATV system includes a head-end facility which provides system program signals over a distribution network to system subscribers. The CATV network can provide a direct communication path in a multi-terminal communications system between the master control terminal which may be located at the head-end facility and the remote terminals which are located at subscriber sites.
As is established in the Oakley, et al. U.S. Pat. No. 3,886,454, a CATV distribution network is designed to most efficiently carry signals from the head-end facility to the subscribers. Normally, the losses in the paths from the head-end to the subscribers are equalized in order to equalize the signal levels received at the subscriber sites. On the other hand, on the return paths from subscribers to the head-end, the path losses are not equalized. The result is that when reception of signals transmitted from remote terminals is considered from the perspective of the head-end a wide variation in the level of received signals is encountered. The system of the Oakley, et al. patent does provide a means for reducing return path noise and equalizing return path signals at various points in the network. However, the Oakley system places those means intermediate the head-end and subscribers which reduces the system reliability. Further, it does not provide for direct setting of subscriber signal levels by the head-end which is desirable to prevent intermodulation distortion between return signals before they reach the equalizing means, and to provide fine tuning of the levels within a narrow range.
The establishment of reliable and rapid communications between the remote terminals and the master terminal requires the reception by the master terminal of return data signals from the remote terminals having an average level greater by some margin than the average level of the system noise, but not so great as to create intermodulation distortion in other signals sharing the return path. Moreover, the levels must be equalized in order to reduce the required dynamic range and accompanying response time of the receiver in the master terminal. In the past, the signal levels of remote terminals have been set when a remote terminal is installed, the setting being accomplished by an installation team comprising two technicians, with one technician at the remote site manually setting the level of the subscriber terminal transmitter according to the instructions of the other technician at the head-end who monitors the received signal level. Once set, the remote terminal's signal level in such a system can only be changed in the same manner. Thus, when the conditions of the network change, it is often necessary to dispatch the team to reset the levels of a number of affected remote terminals in a particular area of the transmission network. It is apparent that the provision of an automatic means for setting the transmission level of a remote terminal is highly desirable, both from the standpoint of maintaining an efficient and reliable transmission network which responds quickly to changes in its own transmission characteristics, and reducing the manpower necessary to maintain the network.
The art of automatically adjusting the level of a remotely transmitted signal in a two-terminal system is well established. For example, Boyer U.S. Pat. No. 3,732,496, Tomizawa, et al., U.S. Pat. No. 3,278,850, Young, Jr. U.S. Pat. No. 2,678,998, Ferguson, Jr., et al. U.S. Pat. No. 3,315,164, and Scarla-Neilson U.S. Pat. No. 4,261,054 all teach systems for the setting or change of the level of a remotely transmitted signal. However, these systems are adapted to be used in atmospheric broadcast systems wherein the transmission characteristics are the same in both directions. Moreover, the taught systems, in being directed to two-terminal systems, allocate a higher proportion of time to level-setting activity than can be tolerated in a multi-terminal system wherein a single master terminal must communicate with a large number, for example 10,000, of remote terminals.
It is well known in the art of electronic system design that various functions and activities in bi-directional, multi-terminal communications systems may be controlled by the use of unique, dedicated signals which are generated in the system for specific purposes such as controlling system amplification, restricting system access, and monitoring transmission continuity. Examples are given in Freen U.S. Pat. No. 3,064,195, McVoy U.S. Pat. No. 3,684,823, Gnagi, et al., U.S. Pat. No. 3,665,461, Atkinson, et al., U.S. Pat. No. 3,697,984, and the Stewart patent cited hereinabove. However, none of these references disclose a system for setting the level of signals transmitted by remote terminals.
Examples of devices which perform remote adjustment of transmitted signal levels are found in Watanabe U.S. Pat. No. 4,205,269 and Kowalsik U.S. Pat. No. 4,319,184. However, as both devices depend for their operation on electro-mechanical components, they are relatively slow-acting and inappropriate for use in a highspeed, multi-terminal communications system.
Self-actuated level setting in specific equipments is taught in Moegen, et al., U.S. Pat. No. 4,281,260 and in Haynes U.S. Pat. No. 2,434,155, however, neither reference is directed to the problem of remote level setting in a multi-terminal communications system.